I have a considerable obsession with the Slenderman mythos and for that reason have studied the being in unnecessary detail when considering it is a completely made-up mythological being. However, for those as interested in it as me, here is a blog in which I detail everything I learned from my research, and in no small part to prepare everyone for the movie I am making.
Slenderman is a mythological being, identified by its adoption of regal garbs for the appropriate time era, long slender inhuman limbs, tendrils that serve as primary appendages, and most importantly of all, a complete lack of facial features.
Slenderman seems to prey upon children, with reports as to what happens with these children remaining inconsistent. Most go missing without a trace, but those that are found can have a variety of mutilations and deformities, with its apparent calling card of leaving them impaled upon trees. Some cases report the children reappearing years later the same age as they disappeared, but in altered states and always dead, suggesting a form of time travel.
This is supported by the number of reports that seem to involve warps in space around Slenderman occurences as well as Slenderman's ability to teleport. Slenderman could only teleport if it could bend space, which would also mean it possesses the ability to bend time, which scientifically go hand in hand.
It's unclear why Slenderman chooses its victims, but the pattern that has surfaced from numerous documentations of the creature's attacks have led to me to a solid theory on why it attacks the way it does. For whatever reason, Slenderman needs its victim to come to it willingly.
This may explain why it chooses to attack children most often, and why encounters with adults are much less subtle and much more terrifying. Children are the ideally naive victim of any abductor because of their trust in adults. Slenderman is known to present itself in a friendly manner towards children, and the way it obviously wears regal clothing proves it is trying to instill some sort of impression on the victim. In children, seeing a business suit creates the impression that the adult is responsible, serious, and straightforward; they'd be very welcoming of such a person.
On the other hand adults seem to only become victims when they've had a past experience with Slenderman; whether that means they've had a child they were related to abducted, or simply decided to research the being. This suggests a telepathic to victims, especially when considering EVPs(electronic voice phenomena) associated with Slenderman's presence and voices that some victims hear. The telepathic link also is supported by the selectivity of those who can see the Slenderman. It's not clear what makes some Slender-sensitive, but it may be linked to the criteria for Slenderman's prey. What is clear is that those who are Slender-sensitive must take heed not to attract Slenderman's interest.
As stated before, evidence suggests Slenderman needs its victim to come willingly, but in adults, this process appears to be a lot more complex. Slenderman has a notorious habit of "playing with its food;" stalking, tormenting, and alarming the victim in many ways to weaken their constitution and find more weaknesses in them. Eventually, Slenderman narrows in on your weakness; most often dedication or curiosity, and uses it to lure you into welcoming it.
The most detailed evidence of this is displayed in its thrall structure. Slenderthralls are minions that help in the torment of victims in some way that Slenderman cannot physically do itself. Sleepers are known for being enthralled only in their sleep, having no recollection of their activities at night, and most often being victims of Slenderman.
Beserkers are essentially zombies to the Slenderman's bidding, and it is currentl unknown where these beings come from, but most likely it involves the Sleeper process, with these Beserkers being the night-time state of a Sleeper.
Agents are willing followers of Slenderman apparently under a semi-organized group or cult, unofficially working together and often guised as Government workers, officials, or authorities, following Slenderman's abuse of class systems and trust.
Revenants are the most dangerous of the classes. To meet one is a true test of will-power. Revenants are documented as past victims returned to torment the victim who is always a family member or friend, and through curiosity or devotion, lure them straight into Slenderman's clutch. They are rarely used and will stop at nothing to accomplish their mission, desplaying the same space-bending and memory altering abilities as Slenderman. If you ever come into contact with one, under no circumstances should you trust it.
The only viable theory as to why Slenderman abducts humans is to use them to abduct more humans, however this pattern only seems to occur in adults and so cannot be confirmed in any way. For now all we can do is avoid its grasp, as we have no way of knowing what it is Slenderman seeks.
The best way it seems to avoid the Slenderman's attention is to simply avoid anything to do with it altogether. If you find yourself the victim of chance however, and Slenderman has selected you, do not panic. Slenderman can only take you if you let it get to your head, and will torment you until one of you relents. Slenderman does seem to give up on those who resist it. Be warned that this is no easy task in the slightest, as Slenderman even seems to return with new tactics if you've defeated it before. If you've read this study however, hopefully you will stand a better chance at resisting when it strikes.