Tenses can often be a sticking point when learning a new language because they don't always line up in exactly the same place on the timeline in both languages (i.e. mother tongue and target language), however it is this concept of a timeline that I want to use to try and explain the difference.
Imagine a timeline first of all and concentrate on the section of the timeline that represents everything that has happened before the present moment. Now, if we take the passé composé, this tense should always appear as a single dot on that timeline, representing a single moment in time where one specific action was completed. The imperfect on the other hand is represented by a section of time in the past whereby an action is not undertaken and completed at a single point, but rather spans a length of time (in the past) or an unspecified amount of time.
Now, let's go back to the example sentences from above:
Passé composé: "Vous avez vu comme Isabelle était surprise en arrivant? Elle n'était vraiment au courant de rien. Oui, c'etait une bonne idée de lui reserver la surprise." Here we are asking if you saw (at one specific point in time) her arrival. Your seeing and acknowledging of her arrival is an action that was completed then and there when she arrived.
Imperfect: "Vous avez vu comme Isabelle était surprise en arrivant? Elle n'était vraiment au courant de rien. Oui, c'etait une bonne idée de lui reserver la surprise." Here we are talking about actions that took place in the past and lasted for either a long period of time, or an unspecified period of time. Isabelle's surprise likely spanned longer than simply the moment she walked through the door; her not knowing about the party also spanned an unspecified period of time, while the good idea existed between the friends for a period of time before the surprise took place.
The difference between the passé composé and the imperfect in essentially the length of time and specificity of the action and its completion.
Let me know if this explanation has help clarify the difference; I certainly hope it has.