Interest rates remain unchanged for the entire duration.
Because the term is fixed for 30 years the rate that a lender will charge is a little higher when compared to shorter terms. The trade off is security but at a higher price. How long you intend to live in a particular property plays an important role in deciding what loan you should take. Fixed rate loans with shorter terms are also available. The advantage is that you pay off your loan sooner and the interest rate is lower than the 30 year program.