Learn Italian: Dicevo la verità, non stavo arrestando un innocente! - I was telling the truth, I was not arresting an innocent person!

Both mean "I was arresting" in the past. However, using stavo (the imperfect of stare) plus the gerund arrestando strongly emphasizes that you were right in the middle of the action at that exact moment. This is called the past progressive.

Why does the first part use dicevo instead of stavo dicendo?

In Italian, the regular imperfect tense (dicevo) is often enough to mean "I was telling". You could absolutely say stavo dicendo la verità, but mixing the two forms is very common. The speaker likely uses the progressive form for the arrest to really highlight that specific dramatic action.

How do you make the -ando ending for arrestando?

To form the gerund (the "-ing" word) for regular -are verbs like arrestare, you drop the -are and add -ando. For -ere and -ire verbs, you drop the ending and add -endo instead.

Why is there an article before innocente if it is an adjective?

In Italian, it is very common to use an adjective as a noun to refer to a person. By putting un (an) in front of the adjective innocente (innocent), it simply becomes "an innocent person".