An intelligence should be able to actually understand its input / output (rather than appear to understand), be able to reason about itself recursively*, be able to learn and generally have state, be able to have an internal thought process without being prompted**.
Some emotion isn't strictly a requirement but likely a desirable quality for human interaction.
* "Gödel, Escher, Bach: an Eternal Golden Braid" and "I am a Strange Loop" are meant to cover this but I've only started the latter
** A sci-fi book series called Old Man's War features a race of aliens that are intelligent but not sentient due to not having an internal thought process and self awareness, which does raise the question of whether it's possible to have one without the other.