The greatest usability cost benefits can be achieved by understanding what users need to do to complete their tasks efficiently, effectively and satisfactorily and then explicitly designing how the user and the system need to interact. This is the work of an interaction designer and precedes screen design and placing buttons and fields on a screen.

Interaction design is the design and definition of user and system behaviours to achieve specific user goals in a specified current or future context of use.

The main steps and principles in designing interactive systems are:

1. Understand the domain including:

• business goals and functionality to be provided
• system capabilities and limitations
• time and resource constraints

2. Understand the users

• understand user goals and tasks and the current and futures contexts in which the system will be used
• place the user at the centre of the design process

3. Do a high level design

• define a framework or model for how the system should optimally behave
• document the user interaction model as appropriate in paper and electronically
• validate the model
• make informed user centric design decisions and trade-offs

4. Do detailed design

• fill in the UI details during detailed design
• validate the detailed design

5. Evaluate and iterate

• iterate the design based on learnings from user and stakeholder reviews
• continually refine each design output until the system meets all requirements

Projects often fear that validation and iteration will increase costs. The reverse is actually true.