engineer

The engineer makes decisions based on his/her learning and the needs of the project.  A good engineer is also creative to come up with new ideas/alternatives.  Some types of designs or methods of support require creativity.

The engineers status (leader/follower) is different on different projects.  They can be a leader during the design and build phases if they have a strong input in the design. However, they aren't always brought in before a final design is reached.  Even if engineers aren't brought in earlier, they can still have a strong influence by making suggestions.  If the engineer isn't creative and only negates designs that won't work or if the architect won't listen to critisism (it must be done this way, so make it work!) then the engineers role is greatly diminished and they are more follower than leader during the whole process.

The engineers influence changes just like their status.  They can influence the whole design, certain areas of the design, or just the hidden structure.  They can influence shape, materials, and layout depending on where supports need to go, how much support is needed, and how much support different shapes and materials provide.