knowledge
You adjust your language based on what you believe your audience already knows. You’d explain a complex concept differently to a child than to an expert.
relationship
Your relationship with the audience (friend, family, boss, stranger) dictates the level of formality, the type of vocabulary, and the tone you use.
status and power
If there’s a power imbalance (e.g., teacher-student, employer-employee), the person with less power will typically use more formal and deferential language.
how many
Speaking to a large group often requires more formal language and a more structured delivery than speaking to a single person.
expectations
The audience’s expectations about the communication, maybe a formal presentation, maybe a casual chat.
other things
Demographic factors like age, gender and others.