Formal:
* Attentive: This is the most direct and common word.
* Heedful: This emphasizes paying close attention and taking notice.
* Eager: This suggests a genuine interest in what's being said.
* Engrossed: This implies a deep level of focus and immersion in the conversation.
Informal:
* All ears: This is a casual way to say someone is listening intently.
* Hanging on every word: This emphasizes the listener's complete focus.
* Riveted: This implies being captivated by what's being said.
Other options:
* Sympathetic: This implies understanding and empathy for the speaker.
* Understanding: This suggests the listener is able to grasp the meaning of what's being said.
* Thoughtful: This implies the listener is considering what they're hearing.
The best word to use will depend on the specific context and the level of formality you desire.