There are several types of films that you can do. Non-fiction documentary films use stock footage often more often than narrative films. This is done because documentary films often cover a range of real topics and will have to reference different events, areas, or items throughout. These are used to paint a general picture, relate the topics being discussed, and generally provide a visual representation. Documentary filmmakers simply cannot get every piece of actual footage they need so stock footage is used often. Stock footage is nondescript in nature, which means that it is not intended to show a specific event. If it did accurately report a specific event then it could be considered news reel footage. Instead it is supposed to show something similar to what is being discussed so that it is taken smoothly by the audience. For example, if an interview subject is discussing an environmental issue and briefly mentions the possible affects of the animal life in Kenya it is appropriate to use stock footage of animals in Kenya. Stock footage is usually not used to give a visual representation of an actual event. If a very specific robbery is being discussed and stock footage of a similar event is used then this would be dishonest as the audience would automatically assume that the footage being shown was accurate to the situation. Use these stock footage clips mainly as B-roll to cover interview clips and try to keep them topical to what is being discussed. Since documentary films are collages of different types of footage you will not have concern yourself with the fact that this footage may look much different than that of the rest of your film.