If one parent is English and the other parent is French, it is a good idea to maintain the experience in the home environment. This means the French parent would only speak French to their child and the English parent would only speak in English. The parents can speak in either language to each other.
In addition to building the parent-child relationship, it also builds the language and cultural aspects which is an important part of the connection you have with your child.
Where dual-language is not possible in the home (i.e. you both speak English), there is still a great deal your child will benefit from in terms of developing their cognitive, memory, problem-solving and decision-making skills.