Why couldn't people be influenced to do things they can already do?
If media has any effect at all on the way we think, and I kind of hope it does, that's what it would affect most: the things under your control. Seeing a man fighting a dragon won't make you more likely to fight dragons. Seeing Rambo shooting up a bunch of foreigners probably won't make you more likely to go looking for armed foreigners to kill- it's more physically possible than fighting dragons, but almost as far outside normal people's everyday experience.
Seeing people drink Coca-Cola, on the other hand... well, there's a reason Coca-Cola pays movies for product placement. If tobacco companies weren't on the run from all the watchdog groups mad at them, they'd probably do the same.
Obviously, all sensory input to your brain influences your thoughts. A dog can be conditioned to automatically salivate upon hearing a bell, and a human can be similarly conditioned to automatically think of beer whenever you mention partying. It's why we have advertising.
You are examining this as if media causes conscious thoughts, ie- "I have seen men shooting other men on screen, so I will shoot men myself". That's not really how it works, any more than a person sees beer commercials and consciously
concludes that since he has seen this in commercials, it must therefore be true that partying must always be associated with beer. It's all subconscious.
If all pornographic and Hollywood sex scenes invariably
showed the guy putting on a condom, this wouldn't necessarily cause people to consciously decide they need to use condoms, but it would create a very strong association, ie- that this is just something that feels right to do before sex.
Of course, it would also be bullshit, because while some people do use condoms before sex, plenty do not, nor should they be asked to. Couples in committed long-term relationships, for example, generally don't use condoms. Showing married couples invariably slipping on condoms before sex is just as ridiculous as 1950s TV shows in which married couples slept in separate beds separated by at least six feet.