doing playing

[Alanmo]

So what is the 81-year-old Paul Newman doing playing a crusty old automobile with blue eyes for headlights in the animated film "Cars," a cartoon set in the world of race cars.
What function does " so " play there ?
Is there a word missing ? which is " IS " ?
So what is the 81-year-old Paul Newman doing IS playing
"Cars," a cartoon set in the world of race cars. Is there any words missing between " cars " and " a cartoon"
I think it is " which is"
"Cars," WHICH IS a cartoon set in the world of race cars

[Casiopea]

What function does "So" play there?

It introduces a question.

Is there a word missing? which is "IS"?
So what is the 81-year-old Paul Newman doing IS playing

Adding 'is' would make the sentence ungrammatical. Consider these like forms:

What are they doing watching TV? They should be studying!
What are you doing with my wallet? Are you trying to steal it?
What are you doing to the carpet? I just washed it. Take your shoes off!

[Are] there any words missing between "cars" and "a cartoon". I think it is "which is"

You're right.

[Alanmo]

Thank you very much Casiopea,
Is the period or question mark after " the race of cars " ? I think it is a question mark after reading your examples.
So what is the 81-year -old man Paul Newman doing playing............, a cartonn....a race of cars ?

[Casiopea]

"So" introduces a question. What are your thoughts?