(covers information from several alternate timelines)
Temporal Cold War
Silik took his orders from a mysterious humanoid figure who communicated from the 28th century. In exchange for doing his bidding, this individual provided the Cabal with the technical expertise necessary to genetically enhance themselves. These enhancements included shapeshifting, enhanced senses, and the ability to survive in extreme environments. These enhancements gave Silik a distinct advantage over those he opposed.
Encounters with Enterprise
In 2151, Silik sent two of his soldiers to retrieve Klaang, a Klingon courier who carried proof of the Cabal's plot to destabilize the Klingon Empire. Later, as Captain Archer of Enterprise NX-01 was attempting to rescue Klaang from a Suliban helix, Silik engaged in a fight with Archer, nearly killing him. (ENT: "Broken Bow")
As punishment for his failure to start a civil war in the Klingon Empire, the Cabal's benefactor had Silik's enhanced vision removed, but promised to restore it if he succeeded in his next mission. Silik infiltrated Enterprise and attempted to steal a Temporal Observatory belonging to Daniels, a temporal agent from the 31st century, and deliver it to his master. Silik confronted Captain Archer and attempted to convince him that Daniels was, in actuality, attempting to alter history, and that Silik was attempting to stop Daniels. Silik later shot Daniels with a particle weapon, apparently killing him. He then took the observatory and attempted to flee the ship, but Archer destroyed the observatory rather than let it fall into the wrong hands. Silik then escaped by jumping out of one of Enterprise's launch bays and free-falling into a waiting cell ship. (ENT: "Cold Front")
The next year, the Cabal attempted to frame Enterprise for the destruction of the Paraagan II mining colony. The Enterprise crew, however, was able to obtain proof of the plot by stealing three data disks from a cloaked Suliban stealth cruiser. On orders from his superior, Silik then led a fleet of cell ships after Enterprise to capture Captain Archer, threatening to destroy the ship if Archer did not turn himself over. Having no choice, Archer agreed, but was then transported to the 31st century by Daniels, who was attempting to protect Archer. Daniels' action had disastrous consequences, however, in that removing Archer from the timeline changed history in such a way as to destroy Daniels's own civilization. This also had the effect of removing the Cabal's benefactor from the timeline, a fact that Silik discovered when he unsuccessfully attempted to contact him. Silik eventually took a device from Lieutenant Malcolm Reed that he'd removed from Daniels' quarters and claimed could allow Silik to reestablish contact with his benefactor. However, this was a trick and when Silik used the device on a temporal chamber, it enabled Archer to return to his own time period. Archer captured Silik and used him as a hostage to force the Suliban to stop their attack on Enterprise. Archer took an unconscious Silik back to Enterprise with him on a cell ship and told his crew that by the time Silik woke up, they would be long gone. (ENT: "Shockwave", "Shockwave, Part II")
In 2153, after Earth suffered a devastating attack which resulted in the deaths of seven million people, Silik abducted Archer from Enterprise under orders from his master and delivered him to a temporal chamber. There, the Cabal's benefactor provided Archer with intelligence that the Xindi were behind the attack, and that they were working for another faction in the Temporal Cold War. This gave Starfleet the information it needed to mount a mission to stop the Xindi from destroying Earth. (ENT: "The Expanse")
One year later, Silik was assigned to stow away on Enterprise, when his master learned that Daniels would be sending it back in time to 1944 to stop the Na'kuhl, another Cold War faction, from constructing a temporal conduit. Wanting the ability to physically travel through time rather than just communicate through it, the Cabal's benefactor ordered Silik to steal the conduit's specifications and deliver it back to him. Silik was discovered, however, and eventually teamed up with Captain Archer in order to stop the Na'kuhl. They were eventually successful in destroying the conduit before the Na'kuhl had a chance to use it, but Silik was shot to death in the process. As he lay dying, Silik admitted that he would have preferred to die fighting Archer but considered the captain a worthy opponent nonetheless. (ENT: "Storm Front", "Storm Front, Part II")
The decision to feature Silik as a recurring character on Star Trek: Enterprise was made even before casting for the part began. A character breakdown sheet which Paramount sent to various talent agents, upon seeking an actor to portray Silik, made clear that he was to be a recurring character and described him as "Late 30s to early 40s. Alien. Physically agile. One of the leaders of the Suliban, a deadly species obsessed with genetic enhancement. Our villain."  Silik was later made older, with the script of "Broken Bow" describing him as "in his fifties." 
Despite having played numerous parts in Star Trek before, John Fleck had to audition for the role of Silik. "They brought me in to audition because I guess they knew it was possibly going to be a bigger role and they wanted just to make me work a little for it," he stated, with a grin. (Star Trek Monthly issue 91, p. 32) Right from the start, Fleck was interested in the part. "I really wanted the role," he admitted, "because I knew it could turn into so much more." (Star Trek Monthly issue 110, p. 30) To prepare, Fleck was given only his sides, rather than the whole script of "Broken Bow". (Star Trek Monthly issue 91, p. 32) "I remember reading the last line on the script and saying 'goin' [with New York accent] instead of 'going'. And I stopped dead and said, 'No, Silik wouldn't say it like that. Keep the camera rolling and I'll do it again.' It got a laugh out of them, and because they see so many people, it's good that they remember you for some reason or another." That Fleck had experience working under prosthetic makeup probably worked in his favor too, the actor subsequently commenting, "I'm sure that didn't hurt my chances." Although the audition was intensive, Fleck wasn't required to screentest with Archer actor Scott Bakula to check if their on-screen chemistry was right. (Star Trek Monthly issue 110, p. 30)
John Fleck didn't know, when he did the audition, that Silik would become a recurring character. At one point thereafter, he stated, "I had no idea that it would be a recurring role." (Star Trek Monthly issue 91, p. 32) However, Fleck also clarified that Silik was someone who was "likely to be an ongoing character." (Star Trek Monthly issue 110, p. 30)
Initially adopting the role of Silik wasn't comfortable for John Fleck, so he had a few early reservations. "When we first started out," he recalled, "wearing those contact [lense]s and the costume was very uncomfortable – all I did was complain." The makeup was not particularly flattering, either. "Somebody saw a picture of me in this greenish light and said I looked like I was covered in bleu cheese," Fleck stated. (Star Trek: Communicator issue 139, p. 41)
John Fleck was given some advice on how Silik was to be depicted, though the actor also came up with some thoughts of his own about the alien. " I remember them saying there should be a reptilian quality to Silik [....] I saw him as someone who will do anything to become the superior race in a way," he explained, "someone who will sell their soul to the devil to get power." More specifically, Fleck believed Silik's devotion to his future-residing benefactor was "like selling his soul to the devil." (Star Trek: Communicator issue 139, p. 41)
Silik's name was not established until his second appearance, "Cold Front". The script of that episode made clear that he used "his Suliban abilities" to alter his face into that of a Borothan pilgrim. Also, in unused dialogue from the same script, the disguised Silik conversationally asked to see the engineering room of Enterprise, which lead to him and the other pilgrims visiting Engineering. In the final version of the episode, it is unclear who suggests this idea.
John Fleck considered it "tough" to figure out whether, in "Cold Front", Silik is being honest with Archer about the future or if he is merely manipulating the captain for his own goals. Although "Cold Front" Director Robert Duncan McNeill repeatedly advised Fleck in one particular scene to play Silik not as an evil character but as if he is being truthful to Archer, McNeill was also uncertain himself about Silik's honesty or lack thereof, rhetorically asking Fleck during production, "Is this the truth? Who knows?" Moreover, the portrayal of Silik in "Cold Front" had to be significantly altered. "I did hear afterwards, from someone, that the powers-that-be higher up felt that it was too nice, and they needed more of an evil edge in there," Fleck acknowledged. "We tried to compensate for that in a couple of places to make me a little more threatening, to get me back in that position." (Star Trek Monthly issue 91, p. 32)
Liking Silik's ambiguity, John Fleck was subsequently eager to see the character return. "We're still trying to find this character," the actor mused. "And if he is a chameleon-type character, a shape shifter, I would think he would be able to change his moods and to become different personas, not always the same arch-villain. His personality is chameleon-like, too. I hope it does get to recur and evolve [....] I'm just waiting to hear when, [how] and if they need me [....] I'm just assuming, based on [the 'Cold Front'] script, that there is a possibility that this character will come back." (Star Trek Monthly issue 91, p. 32)
After appearing in "Cold Front", John Fleck still found discomfort with Silik's Suliban makeup design. In fact, he stated, "I'm hoping some day they'll let me be a human, so I'll not have to wear that make-up for a day." (Star Trek Monthly issue 91, p. 32)
Following his work on ENT Season 1 finale "Shockwave", John Fleck was completely uncertain about what would happen to Silik but remained enthusiastic about reprising the role. "Now that I can see all the possibilities and all the perks, I'm happy to put on the contacts!" he exclaimed. "I'm hoping to keep Silik going as long as possible." (Star Trek: Communicator issue 139, p. 41)
After appearing in ENT Season 2 finale "The Expanse", John Fleck continued to relish his recurring appearances as Silik. "I'm just very thankful every time they hire me and I love being in the season finales," he remarked. "If that's all I ever do on Enterprise, then that's great. It's that old expression – 'keep 'em wanting more.'" However, he still regarded the make-up as a burden. "4am make-up calls are not always that fun," he yawned. On the other hand, he believed that, since he had to wear a tight red jumpsuit for the role, it was fortunate he was slightly skinnier than when he first played Silik. (Star Trek Monthly issue 110, p. 30)
If Silik was to appear after "The Expanse", John Fleck hoped the character would be presented as extremely versatile. "I do hope the writers and producers realise the full potential of Silik. He's a shape-shifter, so why can't I shift into [Seven of Nine actress] Jeri Ryan one episode, or even into Scott Bakula? That way, Scott could do scenes with himself!" Fleck joked. "Seriously, we've already seen Silik disguise himself as a pilgrim, so I'd like to see more of that as he messes about with people's minds. There's so many possibilities, but maybe the budget restraints stop too much shape-shifting." (Star Trek Monthly issue 110, p. 32)
Silik's Human disguise in "Storm Front, Part II" was actually John Fleck without the prosthetics usually associated with the part. This marked the first (and only) time in Star Trek that John Fleck has appeared without prosthetics, as all of his previous roles were aliens as well.
John Fleck was thrilled that, at least with fans of the show, Silik became a well-known Star Trek character but thought doing signings for kids who came to see him with memorabilia featuring Silik was "such a weird experience." Generally regarding the fame Silik brought him, though, he concluded, "There has to be some recompense for wearing all that make-up." (Star Trek Monthly issue 110, p. 32)