3. The information in the statement must be related to your case. Although technically it is the witness' statement and the person can write whatever he or she wants, it's your job to keep the person focused on the events at hand and to his or her level of participation in that event.
Obtaining a Good Statement
Set-up is everything. Ideally you should wait until emotions have calmed down and you are able to focus your time on the person about to give the statement. It is my recommendation that you not multi-task when doing so. Stay with the person writing the statement and answer any questions he or she may have. Proofread the statement before the person signs it. You can't tell people what to write, but you can advise them of anything they left out during your interview. Here is a real-world example of what not to do.
I was working with a deputy who was one of those guys that would hand out the statement form, give the victim a few instructions, and then leave the person alone while he went off to process the scene or to do any other part of the investigation he had yet to complete. On a residential burglary, he gave the victim the standard "Here is how you fill out your statement" speech, checked for the victim's understanding of the task, gave him the form, and went off to process his scene by lifting fingerprints and taking photographs.
About 20 minutes later, he came back to check on the victim, and asked if he had finished with his statement. The victim said he had and gave him the form. Here is what it said: "Came home, stuff gone." Obviously, that was not the intended result and the deputy had to stay with the victim as he wrote a better statement. The point is, you need to treat taking a statement seriously and give it the attention it deserves.