be a bully.
Imagine for a moment, you and your protagonist are walking down a road, let's call the road your screenplay. As you walk down the path, you give him one good nudge to get him startled. This pushes him off the path and takes him a minute to recover. You feel bad, so for the rest of the path you may want to nudge him/her, make him trip a little, maybe laugh at him, but because you are such good friends you don't want to cause serious bodily harm to your protagonist, until the END of the path, when you give him one more push to get him to the end.
This is how most people write screenplays. Yes, their main characters go through some issues, and they have some laughs, or cries along the way. But they are never truly challenged in a meaningful way, and they do not have high conflict throughout the script. For the most part, a protagonist is challenged with a MAIN CONFLICT, through an inciting incident.
It is not enough to have a main conflict that a character strives for throughout the movie. A script must also have lots of hiccups along the way. Not just hiccups, life changing events from which the characters must recover. This will lead, undoubtedly, to the next life changing event that will mold the characters more. However, in each event, the character find strength, and more importantly the audience latches on for more time, the suspense builds, and the hero is humanized.
Since I just read a column about Die Hard, I'll use that as an example. In Die Hard, the main conflict was stopping the terrorists and rescuing the hostages. The inciting incident happened when the terrorists took over. This is the SHOVE your character gets off the path. John McClain must NOW save his wife, rescue the hostages, and stop the terrorists.
But, what if he spent the whole movie pursuant to that goal, without any other conflict building? What if his entire plan was to stop the terrorists at the top of the building, and he faced hardly any conflict until the last ten pages. He made a bomb, or stole a gun, or some little things, but his STAKES were not high enough to cause him to be in imminent danger? Crappy movie, right? Who would watch it?
To go back to my analogy, which will now make sense, Jeb Stuart constantly PUSHED John off the path. Whenever he was back on the path, he was shoved harder, and harder, and harder, until finally, at the end, he was victorious...until Die Hard 2: Die Harder.
Now, aside from being fun to be a bully, and it being exciting to put people in weird and almost inescapable situations, causing conflict is how we define character. If it had been easy for McClain to succeed, we would learn nothing about him. If we pushed him and he stopped, we would know him as a quitter. However, by giving him increasingly more difficult tasks and seeing that he not only doesn't quit, but he also doesn't die AND, to top it off, he wins in the end, tells a lot about him as a person and as a character.
CHARACTERS are not defined by success, they are defined by the ability to fail, brush themselves off, and try again.
So, BE a BULLY. Push your characters off that path as often as possible. This isn't just action movies, but all movies. ROMCOMS, SLAPSTICKS, etc. They all have the same thing in common. A heroic character that is pushed to his limits, and somehow succeeds through seemingly impossible odds. Part of the FUN of a screenplay is to push your characters to the limits. If Bob is successful, make him homeless, how does he fight back to succeed? Through a hairbrained scheme that doesn't make any sense, of from a series of trials and errors which make him stronger for the ride. Because, after all, when you get to the end of the path, and your character hasn't given up, doesn't the victory seem all the sweeter?