Whenever bilinguals are communicating with others, they have to ask themselves two questions: Which language should they use? And can they bring in their other language(s)?
The first question relates to language choice, and the language chosen is called the base language. The second question concerns whether another language can be brought in or not into the base language. Bilinguals stick to one language in many situations (they are said to be in a monolingual mode; see here) but they are known to intermingle their languages when talking to other bilinguals they feel comfortable with (they are then in a bilingual mode).
Here is an example of the latter. A French family in the US is watching some ice fishermen in the dead of the winter and their son, Marc, is right in the middle of the action. The mother is getting very cold and says to her husband: "Va chercher Marc and bribe him avec un chocolat chaud with cream on top." (The French parts mean "go fetch Marc" and "with a hot chocolate," respectively). In this case, the base language is French and the other language, English, is brought in for short segments. (I will come back to the intermingling of languages in a later post).
Language choice, the "which language to use" question, is an intriguing bilingual phenomenon even though, at first, it may appear to be pretty straightforward. This is so when only one language is possible but it is far more complex, and quite subtle, when several languages can be chosen. Four groups of factors explain language choice: participants, situation, content of discourse, and function of the interaction.
As concerns participants, one important factor is the language proficiency of the interlocutors. Bilinguals usually use the language that will be the most successful for communication. How often have I seen a group of bilingual speakers change over to another language when someone arrives, and change right back when the latter has a side conversation or steps away for a few minutes!
There is also the language one customarily uses with a person. When this "agreed upon" language fails to be used in a particular exchange, e.g. on the phone, the interlocutor may be surprised and may ask why at some later time. Other factors include attitude towards a language, age, degree of intimacy, etc.
Concerning situation, the location of the exchange will be a major factor. In some countries, members of a language minority may decide to speak the majority language in public even though, in private, they use their other language.
The content of discourse concerns the fact that some domains and topics are better dealt with in a particular language. (See my post on this). As for the function of the interaction, we all communicate to achieve something and not just to pass information along to someone else. Bilinguals can choose one language over another to create a social distance, raise their status, request something, give a command, etc.
They can also exclude someone by changing language but this sometimes backfires, as in the following example. A Russian-English student once told me that she and a friend were seated in a park when a round little man sat down on the bench opposite them. Switching over to Russian, one said to the other, "He's like a balloon, he's going to blow up!" They then continued their conversation in English but were startled when, some time later, the man walked past them and said, in Russian, "You see, I didn't blow up!" All bilinguals reading about this little "incident" will probably have one or two examples of their own.
Usually several factors taken together explain a bilingual's language choice, and some factors have more weight than others. However complex though, language choice is a well-learned behavior that takes place smoothly and rapidly. Bilinguals are usually quite unaware of the many factors behind their choice; it is just part of being bilingual.
Reference: "Language mode and language choice". Chapter 4 of Grosjean, François (2010). Bilingual: Life and Reality. Cambridge, Mass: Harvard University Press.
"Life as a bilingual" posts by content area: http://www.francoisgrosjean.ch/blog_en.html
François Grosjean's website: www.francoisgrosjean.ch