Been writing for nearly 8 years, written a few scripts.
1- Watch a film called "Big Nothing". It handles the same "gritty" grain you're sort of talking about.
2- Watch the Dave Chapplle sketch with him and Wayne Brady drving around.
The reason I mention these two things is because they both deal with two characters, one of which is dragged along in a situation her doesn't want to be in by a darker character (in your case, the friend being brought to jail and stuff like that).
3- It's not really an idea. It's interesting, but the idea would be the central core of the entire thing.
Think: What gave you this idea?
Is it a character trying to get home?
Is it the two characters trying to do their own thing, but need each other?
Learn to pitch the film. If other people understand the idea, then YOU understand the idea. Break the idea until you get a one sentence pitch.
"It's a [genre] about [characters][situation]."
4-Comedy is all about pain and tragedy. Have insults, have awkward situations, have fights, have witty banter, have the humor the characters would say (i.e. the stoner just says homophobic stuff to the other guy, the other guy mocks the stoners intelligence). The funniest scenes are always when characters fight or are uncomfortable. Stupidity is the highest form of thought.
5-Think, what could go wrong. Then, what SHOULD go wrong.
I had a gay konga-line behind me once.
I got chased by a peadophile once.
I've had my dick fall out of my trousers in front of my father once.
These are all funny, because they're unfortunate.
It's sounds good man, just one thing. Never say "he then spends the rest of the movie trying to get home". It never does your idea any favours because people will just imagine what u said word for word. They will picture him just walking.
Imagine if someone explains Batman as "oh, he fights crime".
Imagine Die Hard as "oh, its a guy shooting terrorists".
Imagine Harry Potter as "oh, its a kid who goes to wizard school".
All true, but are nothing compared to:
"A man avanges his parent's death by fighting crime in a city filled with psychopaths, criminals, and moral temptation."
"A man must single handedly defeat beat a group of trained killers without the help of the police before they hurt his love."
"A young boy must fulfill his destiny of becoming the greatest wizard ever in order to save the world from his parents' killer."
Hope this helped :)