Should I use formal or informal language in the speaking test?
Ideally, you should use a neutral style of language in the speaking test – nothing too formal or informal. You certainly shouldn’t use any swear words. Also, if you are used to speaking English informally with your friends, be careful about letting terms you might use with them slip into what you say during the test. For example, avoid saying something like “Hi, What’s’up?” to the examiner.
However, one thing you should definitely try to do, is use contractions because these are normal in speech. For example, saying things like “If I had to choose, I’d go on a beach holiday” would count positively towards your band score.
In terms of formal language, don’t try to force it. The interview is intended to be friendly and the topics in Part 1 and Part 2 are about everyday topics which don’t require formal language. In Part 3 the language can be more abstract and possibly technical depending on what you are discussing, however, your style of speech should still be neutral – like when talking to a colleague at work, for instance.