The thesaurus is a very useful tool when roleplaying. In fact, you should always have one handy
(actually, I always go out of my way to get a thesaurus when I roleplay…yes, I’m insane).
The thesaurus will help you choose a wider variety of words. I mean, you shouldn’t keep using the adjective ‘big’ should you?

Example: The big ice-cream truck sold big ice-creams. The man who sold the ice-creams was very big. I am also very big and the ice-cream I bought was even bigger than my head.

Wow. How boring. Now, if we use a thesaurus…

Example: The large ice-cream truck sold huge ice-creams. The man who sold the ice-creams was massive. I am also gigantic and the ice-cream I bought was even bigger than my head.

Remember, it’s ok to use words once or twice but don’t keep repeating it!

When using a thesaurus, don’t go overboard like this:
The megalithic insect had a harlequin wing which was acutely mutilated.
That sentence alone took me 10 minutes to make! I had to keep looking through my thesaurus. Anyway, a simplified version of the sentence is:
The big insect had a colourful wing which was badly damaged.
What I think would be a good version of the sentence is:
The large insect had a colourful wing which was badly injured.
You don’t have to go overboard and change every word.

Dictionaries are so useful when roleplaying. You may find that people who use thesauruses too much will cause you to use the dictionary too much. Dictionaries will help when you don’t understand a word. Don’t be afraid to use it!