What is the difference between "being in" and "being down"

I've come across both sentences and was wondering if there is any difference in their meaning. Here are the sentences where I met them:

To be down:

Give me a call if you're down

To be in:

Who's in for the show tonight?

Can one switch them without any impact as in if you're in and who's down for ?

Edit: a new challenger joins the list, to be up to:

Tell me what you're up to

Also, I would point out that I've read/heard that in America and English is my second language.

Now the main question becomes: Can we interchangeably use "to be in for", "to be down with" and "to be up to" ? Can we also switch the extra words such as "to be in with", "to be down to", "to be up for" ? If you have any concrete example where one or another could be more appropriate/meaningful, please let us know!

Solution 1:

Can we interchangeably use "to be in for", "to be down with" and "to be up to" ?

"To be in for" and "to be down with" are more similar to each other than they are to "to be up to."

When I ask you, "Are you in for the show tonight?", I mean, "Are you in the group of people who are going to the show tonight?"

When I ask you, "Are you down for the show tonight?" that could be short for "Should I put your name down on the (metaphorical) list of people going to the show tonight?"

"To be up to" is used almost exclusively in the present tense. "What are you up to right now?" is much more common than "What will you be up to later tonight?"

"What are you up to?" should not be confused with "What are you up for?"

• "What are you up to?" = What are you doing right now?
• "What are you up for?" = What would you be interested in or what do you feel capable of doing?

Solution 2:

You could substitute "around" for both "in" and "down" in your examples

However, "down" seems to imply that a person is unusually in the vicinity from elsewhere. "in" implies a more specific location and also a sense of deliberate presence. "down" is less specific about where exactly the subject is but is saying that they are in the area.

"up" is also used in the same way as "down", example

next time you are up from London please let me know