Another post highlights the need to explicitly explain to students what is expected of them when engaging in a speaking activity. Since these speaking formats require different skills, it is important to assess them differently as well.
For example, though some sort of spontaneous speaking assignment (i.e. “Ask a classmate about…”) may seem like a novice task, it is actually the most difficult because students have to decide what type of language to use (formal, informal, idiomatic expressions, questions words, etc.). Since this is the case, it is important to give as much direction as possible and the teacher should include directions such as scenarios and topics. Then, the teacher assesses the students on the use of these requirements, rather than simply, and perhaps arbitrarily, grading them on how much they can say.
Regarding transaction (see article on Crafting Speaking Activities) activities, and keeping in mind that the linguistic accuracy is not as important, grading simply on whether or not the task is completed usually works well. A performance activity generally requires some form of writing before the student speaks publicly and the assessment should account for the written piece as well.
The more explicit the expectations and the grading elements, the more likely students are to engage in the process of communicating competently.