Depends on what you mean by professional. There are a ton of ways to make a living playing guitar. If you mean in a band, then I would say – play well with others. Compliment the band and understand what your job is. At every moment of every song there is a focal point. You have to KNOW what that is. 90% of the time it’s a singer singing. Your most important job is to make that singer sound as good as you possibly can. You also need to look good as you do it – without taking focus away from the singer. When it’s time for you to solo – then it’s the band’s job to make YOU sound and look as good as possible.
If you’re doing session work, you need to be as prepared as you possibly can be. Be in tune. Have a great attitude and be ready to read some music. More often than not it will involve nothing more than chord charts, but sometimes there is a specific line they want you to read. Be creative, in case you have to invent your part or they ask you for ideas – which happens A LOT.
I would just add that if you have talent, there are certain things you can fake. The one thing you cannot fake is experience. There is no substitution for experience. The ONLY way to get good at performing is to perform – in front of people. A couple hours on stage is better than a year standing in front of a mirror. Practice as often as you can and then get out and play!!!!