There has been a lot of discussion around writing circles with regards to how a bilingual person would use both of their languages. There are definitely not great ways of doing this, so I wanted to give an example from my own use of language(s) to see if that helps.
English is my primary language, with Spanish as my secondary language. I generally think and dream in English, though occasionally a Spanish-language dream will come my way. I read and write in Spanish better than I speak it, though with practice I can get more comfortable.
Having said all of that, there are certain things that are just easier in Spanish than they are in English. It could be because I heard my mother saying them all the time when I was a kid, and so it became more like a form of family slang than anything else, but some phrases are so much faster and snappier in Spanish than in English. It helps that, frequently, you don't actually pronounce all of the letters in Spanish - there are sort of blank spots where the sound would be.
Examples would help. If I'm behind the wheel of a car, I'm more likely to break out the Spanish than in just about any other circumstance. "A que gente!" just rolls off the tongue more easily than "For crying out loud!" or something similar. There are a lot of phrases in the family vocabulary that I'm not certain are common in general Spanish speaking, but certainly popped up a lot in our house. Most of them began with "A que," which...doesn't really translate. It usually denotes some sort of frustration or irritation. If I get an "A que Stephie!" from my mom, I know she's getting exasperated. Needless to say, I got this a lot as a teenager.
I'm not sure why, but when I talk to animals, I'm more likely to slip into Spanish. A lot of times it's some form of endearment ("niño/a," "perrito," that sort of thing), though sometimes it falls into that exasperation frame again. Ramses, may he disembowel fuzzy mice in heaven, was frequently "gatito hombre," usually when he was on the kitchen counter or somewhere else he shouldn't have been. It just wouldn't have been the same to call him "little cat man," you know?
Anyway. For me, Spanish is useful when it's faster or easier to think of instead of English, but it's not necessarily the first thing I'll come to. I don't know how people who have Spanish as their primary language would treat English - anyone want to weigh in?