Take interviewing a witness, for example. Because of the nature of the job, first responding officers have to get the basic information, handle the call, and move on to the next call as quickly as possible. Detectives, on the other hand, can spend all day with a witness if they think it will help. Though the time restrictions for first responders and detectives might be different, their goals remain the same. Each one tries to do the best they can with their part in the investigation.
Of the two, I believe the first responder's role is more important because the beginning always determines the end. What any first responding officer does or fails to do will ultimately determine the outcome of the investigation. For example, if a first responding officer makes any mistake in obtaining witness information or the handling of evidence, or inadvertently violates someone's constitutional rights, the case can be lost long before it ever gets to trial.
From the Officer's Perspective
The best investigative template I have ever used involves trying to answer who, what, when, where, why, and how. If you can answer all of these questions, you have obviously solved the case and your investigation is complete. On the other hand, if you can only answer a few of these questions, then you have given a detective a great place to start.
As the initial investigator, you are responsible for answering each of these questions in some way. It's a relatively simple task as long as you remember that each question has but three possible answers. You either have the information, you don't have the information, or you don't have it now but can get it later. I'll use obtaining contact information as an example. You are speaking with a complainant and need to get their contact information. You ask for their phone number. The complainant will either give you their phone number, state they don't have a phone number, or tell you they can't remember and will have to give it to you later. In other words, it may not be the answer you are looking for, but there is an answer you can provide as part of your investigation.
If your investigation clearly states what information you have, what you could not obtain, and what needs to be done later, no one should find fault with it. It's an approach all first responding officers should take a good look at throughout their investigation. It makes the pass off to the detective as complete as possible. When you are done, there should be no doubt in anyone's mind where your investigation ends and where the detective's needs to begin.