Pretérito imperfecto vs pretérito perfecto simple
I know generally one whould use Pretérito imperfecto for incomplete past actions, as in it was going on in the moment being described, and pretérito perfecto simple for actions that has been completed. I get the feeling tho that this isn't always the case in spanish.
If so... Any hard and fast rules?
As you have already said, generally speaking, the preterite is used for actions in the past that are seen as completed, however, the completion may not always be clear. Use of the preterite tense implies that the past action had a definite beginning and definite end.
Juan habló de la una hasta las dos - John spoke from one until two o'clock.
(clearly stated beginning and end)
As mentioned above it is important to realize that the beginning and the end may not always be clearly stated.
Juan habló dos horas - John spoke for two hours.
Juan habló con la estudiante - John spoke with the student.
(both these have an implied beginning and end)
Generally speaking, the imperfect is used for actions in the past that are not seen as completed. Use of the imperfect tense implies that the past action did not have a definite beginning or a definite end.
Las chicas hablaban en inglés - The girls used to speak in English.
(no definite beginning or end)
The preterite tells us specifically when an action took place.
The imperfect tells us in general when an action took place.
So even though these two rules seem quite simple in practice they can be a complex topic. Hopefully as our Spanish level improves we learn the finer points of these generalisations.
We had a good game thread going on the Imp vs Pret and although it isn't running at the moment as the moderator is (hopefully temporarily) unavailable at present you can still probably learn a lot from the previous answers.