Sure! Here are the suitable words to fill in the gaps for the letter:

1. **who**
2. **favourite**
3. **His**
4. **much**
5. **eyes**
6. **His**
7. **is**
8. **my**

So, the completed letter would read as follows:

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Hi Harry,

Can you guess who (1) **who** people love watching? I know so (2) **much** about (3) **his** favourite films. (4) **His** favourite film is Pirates of the Caribbean. I think (5) **he** is American. His eyes are brown and (6) **his** birthday is on June 9th. (7) **He** is really famous and (8) **my** favourite actor is? Well, he is really famous and his name is Johnny Depp.

Write back soon!
Judy

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Feel free to ask if you need more assistance!