Both sentences are correct, but they differ slightly in nuance: 1. "That was more interesting than I expected" (Past Simple):
This is straightforward and neutral. It implies you had a certain expectation, and the experience exceeded it. It's commonly used in casual conversation. 2. "That was more interesting than I'd expected" (Past Perfect):
This is slightly more formal or reflective. It emphasizes the earlier expectation (before the event happened) and its contrast with the outcome. It can suggest a bit more thought or surprise. Most natural: The Past Simple ("I expected") is more common in everyday speech because it's simpler and works well for general observations. The Past Perfect ("I'd expected") is often used when the timeline or context requires extra clarity.

Those are two perfect sentences that mean different things.