It is really a challenge to express oneself in a foreign language. However, keep it in mind that a language is a tool of communication. An interviewer will make it clear that you can exchange your own ideas with him in the foreign language without misunderstanding or abrupt break in the process due to your inappropriate word choice and sentence building or even no words to assist you. Concise and logical statement means a good mastery of the language and clear thinking. So mutual communicating is listed on the top of your priority, which is based on your language skills.
Besides vocabulary and grammar, pronunciation and intonation are factors which influence your oral communication as well. Poor pronunciation and intonation, though not as serious as wrong words and sentences in general terms, will affect the interviewer's judgment negatively. After all, the language of English is one of the items put into his account while an interviewer assesses a non-English native speaker.
In a word, a natural fluent oral language, concise and logical statement and clarified vivid rhythmic voice prove your communicative ability.