What were they thinking?

In novels we become spoiled, being able to peer into a character’s mind to know what they are thinking. As a screenwriter, we don’t have that luxury. We must only write what the audience can see or hear. Otherwise, they’re known as “unfilmables” or “invisibles.”

So does that mean we can’t convey a character’s thoughts? Well, yes and no.

No

Elaborate thoughts like these examples are strictly forbidden in screenwriting:

Peter steps out of the elevator. He thinks about his troubled past 
and lack of education.

Josie wonders if she should head back to work or give Brenda a call.

Malcolm secretly wants to kill Ian, but he can't because there are 
too many witnesses here.

They’re unfilmable because it’s impossible to convey those thoughts on screen… unless  you use a voice over. But that’s a script-wide decision and needs to feel organic to the story you’re trying to tell for it to work.

Bottom line, if the audience can’t see it or hear it, you’re not allowed to write it. The script reader shouldn’t have any insight or comprehension advantages over audience members.

Yes

You can get away with writing simple thoughts if — and it’s a big IF — the actor can portray those thoughts on their face so we as an audience can see it.

A disheveled, grungy Sarah shuffles up to the checkout line. Plunks 
her quart of Rocky Road ice cream on the conveyor belt.

The man in front of her turns around -- it's her ex-boyfriend, Chad. 
Fuck my life.

In this case, the actress could clearly portray that thought on her face. When conveying a thought in an action line, italics are often used (arguably the only justification for italics in a script).

Same goes with emotions. You can write things like:

She's mortified.

The horror.

She wishes she could crawl into her ice-cream container.

A good actor can definitely convey those feelings on their face. (No need for italics, in these examples, as they’re not written as thoughts.)

Alternatively

Real talk — using thoughts in a script is an advanced technique. If you have any doubt whatsoever, use more conventional action lines that literally show how the character reacts:

Sarah's eyes rocket wide.

You can even hedge your bet with something that combines an emotion:

Sarah's eyes rocket wide, embarrassed beyond words.

Or one that combines a thought:

Sarah's eyes rocket wide. Someone kill me now.

Just don’t be too specific with how actor’s react. Then you cross into the territory of micro-description.

Also, keep in mind that some newer script readers, with just enough experience to be dangerous, may have negative reactions to the use of such thoughts. You can’t do that in a script! True story.

Once more with feeling

If we can’t see it (external character actions/reactions/displays of emotions) or hear it (dialogue), don’t write it.

Have I confused the hell out of you, or was this helpful? Let me know!