formal
informal
To make it clear - арестовали/"they arrested" is not restricted to informal settings, it's a pretty formal and correct way to say it in 99% cases. Participles like был арестован/"was arrested" are called "канцеляризмы" by linguists, and they are usually used by state officials like deputies, police etc. in their official reports, they have their own clumsy speech, for example, имеет место быть literally means "has a place to be" and means "there is". Except state officials in highly formal reports/interviews, no one really uses participles as often as them.