Will a person be held accountable for what he says aloud to himself when alone?

There is no accountability for mere self-talk, unless it leads to a statement or an action, because the Prophet, peace and blessings be upon him, said: "Indeed, Allah عز وجل has forgiven my nation for what they talk to themselves about, as long as they do not act upon it or speak of it." Seclusion does not prevent accountability if the statement is something for which one is held accountable and the speaker is a legally responsible individual.