"acting director" is usually someone who is working out of title, such as an assistant director who is taking on the duties of a director until a director is found.
an "interim director" is someone who is working in title, but temporarily until a replacement is found.
Usually an interim director is a director from another department who is filling both roles... I.E. Director of finance, interim director of IT.
Sometimes, a department may have a director move to another department while an assistent director becomes acting director...
So, as in the above example, Bob is the director of finance, but the IT department is a mess, and the director was fired. Bob had been in IT in the past, and can serve as interim director of IT. Bob has his assitant director Alice, fill his role while they look for someone to fill the IT spot....
So, you have Alice, as acting director of Finance, Bob as director of Finance, interim director of IT.
EDITED TO ADD:
The difference between "Interim" and "Acting" is that someone who is "Interim" already has a similar title/qualifications and an "Acting" director is someone who doesn't have the title/qualifications, but is ACTING in that role as if they did.
"Acting" be it "Acting manager" or "Acting director", can be used to get experience in a role before a promotion. Example, after six months as "Acting Director of Finance", Alice is promoted to Director of finance, and is no longer "Acting Director"
Also, (thank you Damila) an "Acting" director may not have the pay of a director, while an "interim" director likely will.