Susie Salmon in The Lovely Bones is a lot like Mia Hall in If I Stay. Both characters are kept from participating in the action of the book for most of the story. In Susie's case, it's because she's dead. In Mia's case, it's because she's in a coma after a car accident. Both narrators watch the people they left behind function and try to deal with loss. Both narrators are removed from the story for the majority of the book as decisive characters—though Susie has to decide to move on and Mia has to decide whether to stay or go. Susie is eventually able to move on and Mia chooses to stay alive.
Alice Sebold creates suspense by having characters interact with Susie's murderer. Her father, for example, does a project with him and when Susie tries to communicate the truth, he doesn't understand enough to see the whole picture. However, he does understand that something is wrong and continues to try to find who killed his daughter. Other characters are also affected by Susie's death. Ruth, for example, has a connection with the dead girl and the way it shapes her life creates additional suspense in the novel.
Per eNotes policy, I have edited the two questions down to one: How does the author create suspense in the story?
Suspense in The Lovely Bones is portrayed in an unusual way. What would be the most suspenseful moment in most books with a similar theme, Susie's murder, actually takes place in the beginning of The Lovely Bones. The reader is told in the first paragraph that Susie is a murder victim. This fact is never in question.
Suspense comes from several other sources, most notably Mr. Harvey. The Salmon family doesn't know at first that Mr. Harvey is the murderer, and later they strongly suspect him but cannot prove his guilt. They must live near him; see him every day. This creates a suspenseful atmosphere that climaxes when Lindsey breaks into his house looking for evidence. The reader fears for Susie's sister, because she doesn't know how evil he is.
Every time Mr. Harvey is near another person, the reader fears for that person's safety because we know his true nature and they do not.