The design and construction of a building involves many parties. The Architect is one but many who must ensure that the building fulfils the Clients brief, meets the Design Standards required by the Country/Area where the building is being constructed, meets the budget and ensure that the building can be constructed, used and maintained safely.
The Architect is often the first contact with the Client developing scale and form and more importantly drawing up the Clients Brief. Often a Client does not know what they want from their building. It is the Architect's job to find out what is required and develop a concept. Often Architectural practices have dedicated Architects for this role. Once the concept is designed and the brief defined a 'project Architect' would get involved. It is their job to ensure that all legal design requirements for building are met, produce the specification and construction drawings and ensure details are worked out.
During construction the Client often retains the Architect to inspect the workmanship of the builder, deal with problems that arise from construction and be the Clients contact with the Builder. Modern property markets can change rapidly so it is not uncommon for part of the Clients brief to change mid construction. eg. a floor in a proposed Office building may suddenly be worth more to the Client as apartments. The Architect would then deal with such changes, working with the Contractor and design team to ensure the change can be dealt with without detriment to the construction project.
It is the sole responsibility of the Contractor to be in charge on site. The Architect can request or issue instructions to the Contractor for work to be undertaken but the Contractor ultimately has sole responsibility for constructing the works.
Architect does also have a responsibility for making sure health and safety on site is kept to a high standard.