Gollum and Pronouns

In book four on page 618, Frodo extracts a promise from Gollum to help him and Sam get into Mordor. Frodo asks that Gollum promise by the Ring rather than on it as Gollum had initially wanted to, saying that a direct line of connection to the Ring would be detrimental for Gollum. Eventually, Gollum promises saying, "We promise, yes I promise . . . I will serve the master of the Precious."

This was interesting to me because the quick change from using 'we' to refer to himself to 'I' could be initially seen as a typo, however, later on in the book we see that it is actually a clever way to let the audience know when Gollum was speaking truthfully.
 
On page 643, Frodo notes "that Gollum used I, and that seemed usually to be a sign, on its rare appearances, that some remnants of old truth and sincerity were for the moment on top.

We already know that getting any information out of Gollum is extremely difficult. At the Council of Elrond Gandalf tells how he struggled with Gollum for many days until he got something even remotely helpful out of the creature. Because Frodo and Sam are now relying on Gollum to take them safely into Mordor it is important that we know we, and the characters, can trust him to a certain extent. If the characters couldn't trust him at all then the travel into Mordor would take longer, or they would end up going in through the front gate because they don't trust Gollum has another way, which would lead to their deaths