There are numerous synonyms for the phrase "filling someone in" that can be used in various contexts. Some of the most common ones include briefing, updating, informing, reporting, enlightening, and acquainting. Other expressions that convey a similar meaning include bringing someone up to speed, keeping someone in the loop, telling someone what's going on, clueing someone in, and getting someone up to date. Most of these expressions are informal and can be used in casual or professional settings to communicate information or share important news. Choosing the right synonym depends on the tone, context, and purpose of the conversation.